WO2001061800A2

WO2001061800A2 - Widely tunable laser

Info

Publication number: WO2001061800A2
Application number: PCT/US2001/005078
Authority: WO
Inventors: Vijaysekhar Jayaraman; David Welch
Original assignee: Gore Enterprise Holdings, Inc.
Priority date: 2000-02-18
Filing date: 2001-02-16
Publication date: 2001-08-23
Also published as: EP1256150A2; WO2001061800A3; AU2001241525A1; JP2003523637A

Abstract

A method and system for generating a widely tunable laser output comprising generating a pump optical beam, providing at least one semiconductor laser chip in the path of the pump optical beam such that the at least one semiconductor laser chip emits a tunable optical beam, and moving the at least one semiconductor laser chip relative to the pump optical beam such that the wavelength of the tunable optical beam is controlled by the location of the semiconductor laser chip upon which the pump optical beam impinges. The semiconductor laser chip(s) may be implemented using vertical-cavity surface-emitting laser(s) (VCSELs) each having a resonance that varies with position.

Description

TITLE OF THE INVENTION

WIDELY TUNABLE LASER

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to lasers, and more particularly to a widely tunable semiconductor laser.

2. Background Art

Widely tunable lasers are critical components in many systems, such as in emerging wavelength-division multiplexed (WDM) systems. Existing widely tunable semiconductor lasers may be broadly categorized according to the mechanism employed for providing tuning: (1) mechanical motion to change the length of the laser cavity and/or angle of a diffraction grating (i.e., mechanical tuning), and (2) electronic effects to change the refractive index, and therefore the optical path length of the laser cavity (i.e., electronic tuning).

Mechanical tuning provides wide tuning range and simple wavelength control-typically with one control variable. An example of a mechanically tuned device is a vertical-cavity surface-emitting laser (VCSEL) with a microelectromechanical (MEM) mirror, as described, for example, in P. Wang, P. Tayebati, D. Vakshoori, C.C. Lu, M. Azimi, and R. Sacks, "Half- symmetric cavity microelectromechanically tunable VCSEL with single spatial mode operating near 950 nm, paper PD1.5, LEOS '98 conference, December 1-4, 1998, Orlando, Florida, which is herein incorporated by reference. Such a device is schematically depicted in FIG. 1, and includes a movable mirror 10 separated from active region 12 (which overlies DBR 14) by an air gap region 16. One disadvantage of mechanical tuning is sensitivity to mechanical vibrations, resulting in complicated and expensive packaging. Another disadvantage is the need for a tunable air gap in the laser cavity, which may result in intra- cavity coupling and/or diffraction losses, and compromised laser performance.

Electronic tuning is essentially immune to mechanical vibrations, and the device structure/design for electronic tuning generally does not result in compromised laser performance. Yet, because refractive index changes in semiconductors are small, achieving wide tuning range with complete wavelength coverage requires hopping among many laser modes. See, e.g., V. Jayaraman, Extended tuning range semiconductor lasers with sampled gratings, Ph.D. Dissertation, University of California at Santa Barbara, 1994. Examples of widely tunable lasers using refractive index effects include sampled grating distributed Bragg reflector (DBR) lasers, grating-assisted co-directional coupler lasers, and Y-cavity lasers. These devices have complicated tuning algorithms. Widely tunable lasers using electronically induced refractive index changes typically require three control variables to set the wavelength. In addition, the relationship between control variables and the output wavelength is often not predictable from device to device or over the lifetime of the laser. This unpredictability necessitates including a wavelength monitor with the tunable laser. See, e.g., B. Mason, S.P. Denbaars and L.A. Coldren, "Tunable Sampled Grating DBR lasers with integrated wavelength monitors," Photonics Technology Letters, August, 1998, 1085-1087. Further, these devices are multi-section lasers requiring multiple epitaxial regrowths, which renders the manufacture of such devices difficult, thus reducing yield. Both electronically tuned and mechanically tuned semiconductor lasers have an ultimate tuning range that is limited to the available gain bandwidth in one active region. For example, in indium phosphide (InP) material system based lasers operating at around 1.55 microns, this tuning range is limited to about 50 nm. In summary, existing widely tunable semiconductor laser suffer from, inter alia, one or more of the following problems: complicated fabrication and packaging, compromised laser performance, complex or non-reproducible tuning algorithms, and tuning range limited to the gain bandwidth of one active region.

SUMMARY OF THE INVENTION

The present invention provides such advancements and overcomes the above mentioned problems and other limitations of the background and prior art, by providing a method and system for generating a tunable laser output comprising generating a pump optical beam, providing a semiconductor laser chip in the path of the pump optical beam such that the semiconductor laser chip emits a tunable optical beam, and moving the semiconductor laser chip relative to the pump optical beam such that the wavelength of the tunable optical beam is controlled by the location of the semiconductor laser chip upon which the pump optical beam impinges. In accordance with an aspect of the present invention, a tunable laser system includes a pump laser that emits an optical pump beam of a first wavelength, and at least one semiconductor laser chip upon which the optical pump beam impinges. The at least one semiconductor laser chip is selectively movable relative to the optical beam such that the optical pump beam is capable of impinging on different local regions of the at least one semiconductor laser chip. The at least one semiconductor laser chip generates a second optical beam of a second wavelength in response to the optical pump beam, and the second wavelength is a non-constant function of the local region upon which the optical pump beam impinges. In accordance with an aspect of the present invention, the at least one semiconductor laser chip is a single semiconductor laser chip having a resonance that varies along at least one spatial dimension of the single semiconductor laser chip. Alternatively, the at least one semiconductor laser chip may be an array of at least two semiconductor laser chips, and each such laser chips may also have a resonance that varies along at least one spatial dimension. The at least one semiconductor laser chips may be implemented as VCSEL(s) and/or as edge-emitting semiconductor laser(s).

In accordance with another aspect of the present invention, the at least one semiconductor laser chips may be mounted on a movable stage to provide the motion relative to the optical pump beam, and such a movable stage may include a piezoelectric transducer that is operative in providing the motion.

In accordance with a further aspect of the present invention, the optical pump beam and/or the tunable optical output beam may be guided by one or more optical fibers. In one implementation, a single optical fiber guides both the optical pump beam to and the tunable optical output beam from the at least one semiconductor laser chips. Such optical fibers may be mounted and aligned using one or more v-groove substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional aspects, features, and advantages of the invention will be understood and will become more readily apparent when the invention is considered in the light of the following description made in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a schematic cross-sectional view of a prior art vertical-cavity surface-emitting laser (VCSEL) with a microelectromechanical mirror, which may be implemented in accordance with an embodiment of the present invention; FIG. 2 schematically depicts component parts of a widely tunable laser system, in accordance with an embodiment of the present invention;

FIG. 3 shows a perspective view of various components of a widely tunable laser system that employs a v-groove substrate for mounting optical fibers, in accordance with another embodiment of the present invention; FIG. 4 schematically depicts various components of a widely tunable laser system that employs a single optical fiber for guiding both an optical pump beam and a tunable optical output beam, in accordance with a further embodiment of the present invention;

FIG. 5 shows a schematic cross-sectional view of a portion of a VCSEL having a tapered oxide to provide a spatially dependent resonance, in accordance with an embodiment of the present invention;

FIG. 6 depicts a schematic plan view of the relative location of circular index guides with respect to a cavity-oxidation edge for a VCSEL chip, in accordance with an embodiment of the present invention; FIG. 7 depicts a schematic plan view of the relative location of circular index guides with respect to a stepped cavity-oxidation edge for a VCSEL chip, in accordance with another embodiment of the present invention; and

FIG. 8 illustrates a schematic plan view of a portion of a VCSEL in which the cavity is defined by an index guide along one dimension only and by gain guiding in an second orthogonal dimension, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, there are schematically depicted component parts of an illustrative widely tunable laser system, in accordance with an embodiment of the present invention. Pump laser 22 emits an optical beam 22a which impinges on a local region of any one of semiconductor laser chips 26a1, 26a2, 26a3 . . . 26a(n-1), 26an, which are mounted on a movable translation stage 24, and each of which has a resonance that varies with position along at least one direction in which movable translation stage 24 translates (shown as the x-direction). In response to the incidence by pump beam 22a, the local region of semiconductor laser chips 26a1, 26a2, 26a3 . . . 26a(n-1), 26an generates a tunable output optical beam 26b, its tunability being based on the spatially dependent resonance. Tunable output beam 26b may be coupled via an optional lens element 28 into an optical fiber 30, and may be used for various applications such as for spectroscopy or wavelength-division-multiplexed (WDM) optical communications. A controller 40 provides a position control signal via line 42 to movable translation stage 42, in order to control the local region upon which pump beam 22a impinges, and thus the wavelength of tunable output optical beam 26b. A packaging/substrate element 20 may be employed as a common package/substrate for mounting semiconductor laser chips 26a1 , 26a2, 26a3 . . . 26a(n-1), 26an, and may also include (1) a temperature sensing element (not shown) sensed by controller 40 via signal line 48, and (ii) possibly also a heating and/or cooling element controlled by controller 40. Data may be impressed upon output optical beam 26b (e.g., in a communication system) via an external modulator 32 (e.g., a Mach- Zehnder), or by directly modulating pump laser 22. More specifically, in the present and/or in alternative embodiments, pump laser 22 may be implemented in various ways depending on, for example, the system application, which may require certain laser parameters such as wavelength(s), linewidth, power, pulsed or continuous-wave (CW) operation modes, etc. For example, pump laser 22 may be selected from different laser sources (e.g., solid state lasers, excimer lasers, semiconductor lasers, etc.) and/or may include various laser/laser-optics designs (e.g., etalons, Q-switching ) to provide the desired optical pump beam characteristics. By way of further example, if pump laser 22 is implemented as a semiconductor laser, it may be an edge-emitting semiconductor laser or a VCSEL. Additionally, pump laser 22 is shown as having a modulation input 22b to which a data signal may be input for modulating pump beam 22a, and hence tunable output beam 26b.

It is understood that the wavelength of pump laser 22 is selected to optically pump the laser or lasers that provide tunable output beam 26b (i.e., in this embodiment, semiconductor lasers 26a1, 26a2, 26a3 . . 26a(n-1), 26an) and thus, is less than the wavelength of tunable output beam 26b. Although a single pump beam wavelength satisfying this requirement over all tunable output beam wavelengths may be employed, the output wavelength of pump laser 22 may be varied depending on the resonance that is pumped, which in FIG. 1 would correspond to varying the pump beam wavelength according to the x-position of stage 24. For example, in an implementation in which the tunable output can vary from about 700 nm to about 1700 nm, a pumping wavelength in the approximate range of 300 nm to 700 nm may be used for providing tunable outputs in the approximate range of 700 nm to 1100 nm, whereas a pumping wavelength in the approximate range of 700 nm to 1100 nm may be used for providing tunable outputs in the approximate range of 1100 nm to 1700 nm. Such variation in the pump beam wavelength may be achieved, for example, by using a tunable pump laser configuration and/or by using two or more pump lasers.

It is noted that while pump beam 22a is schematically shown as directly propagating through free space onto semiconductor laser chips 26a1-26an, the optical path traversed by pump beam 22a may include various optical components such as lenses, mirrors, gratings, and optical fibers. Coupling pump beam 22 into an optical fiber is well suited for accurately aligning the pump to the semiconductor laser chips 26a1-26an as well as to any elements for receiving (e.g., lens 28 and/or optical fiber 30) tunable output beam 26b.

In fact, as understood in the art, by packaging optical fibers in v-groove substrates (e.g., ceramic or silicon), accurate alignment may be achieved. In accordance with an embodiment of the present invention, such v-groove packaging of optical fibers may be employed both for coupling pump beam 22a to and for coupling tunable output beam from semiconductor lasers 26a1-26an. Separate v-groove substrate packages may be used for the respective optical fibers guiding pump beam 22a and tunable output beam 26b. Alternatively, as illustratively depicted in FIG. 3, both the input optical fiber 31 and the output optical fiber 33 may be mounted in a single or commonly formed/aligned v-groove 35 on a single v-groove substrate 37, and spaced apart such that semiconductor laser chips 26a1- 26an (only laser chip 26a1 shown for clarity) translate between the ends of the optical fibers in a direction perpendicular to the v-groove length. In the embodiment of FIG. 3, an etched channel region 39 along which the semiconductor laser chips are controllably displaced is provided to, for example, facilitate positioning the semiconductor lasers in the path of the pump beam that emanates from input optical fiber 31. In various alternative embodiments, such a groove may not be necessary. It is noted that a single or commonly formed/aligned v-groove simply refers to the v-groove portions for the input and output fibers being patterned and formed on a common substrate such that they are aligned to each other. For example, a common patterning step and a common etching step may be used to form both v-groove portions. Such a single v-groove substrate configuration provides a simple, essentially self-aligned technique for aligning the optical fibers that guide and receive the pump beam and tunable beam, respectively.

In yet an alternative variation for coupling the pump beam and receiving the tunable output beam, regardless of whether v-grooves are employed, a single optical fiber may be used to guide both the pump beam into and the tunable output beam from the tunable semiconductor laser chips. More specifically, referring to the illustrative embodiment of FIG. 4, optical fiber 41 guides a pump beam (e.g. 980 nm) to a VCSEL chip 49 which outputs a tunable output beam (e.g., 1550 nm) back into optical fiber 41. Optical fiber 41 is coupled to a dichroic beam splitter 43 which is coupled to optical fiber 45 and optical fiber 47. Optical fiber 45 guides pump laser radiation into dichroic beam splitter 43, whereas optical fiber 47 guides the tunable output beam away from dichroic beam splitter 43, which separates the input pump wavelength (e.g., 980 nm) from the tunable output wavelength (e.g., 1550 nm). As may be appreciated, such an approach avoids the need for aligning two optical fibers to each other. Although in the embodiment of FIG. 2, pump laser 22 is fixably mounted and its pump beam 22a has a fixed optical path with respect to the laboratory frame, as described further hereinbelow, pump laser 22 may be mounted on a movable stage instead of or in addition to semiconductor laser chips 26a1, 26a2, 26a3 . . . 26a(n-1), 26an being mounted onto movable translation stage 24, provided that semiconductor laser chips 26a1 , 26a2, 26a3 . . . 26a(n-1), 26an are moved relative to pump beam 22a in a controllable manner such that the wavelength of the tunable optical beam is controlled according to the spatial location of semiconductor laser chips 26a1, 26a2, 26a3 . . . 26a(n-1), 26an upon which pump beam 22a impinges. More specifically, in the embodiment of FIG. 2, since optical pump beam 22a is fixed

(i.e., relative to the laboratory frame of reference) and movable translation stage 24 moves the semiconductor laser chips relative to optical pump beam 22a, the emission location of tunable beam 26b is fixed, which is convenient for various applications. Such a fixed emission location (nor a fixed propagation direction) is not necessary, however, and it is understood that myriad alternative embodiments are possible to provide the necessary relative motion between an optical pump beam and one or more semiconductor lasers upon which the optical pump beam impinges and which provides a resonance wavelength that is dependent upon the position of the semiconductor laser(s) impinged upon by the optical pump beam. For example, in an alternative implementation, pump laser 22 may be controllably moved and semiconductor laser chips 26a1, 26a2, 26a3 . . .26a(n-1), 26an may be fixed or also be controllably moved (e.g., using translation stage 24). In yet an alternative implementation, pump laser 22 may be fixed, but its beam may be optically scanned across one or more semiconductor lasers, which may be fixed or may be controllably moved as well. Despite variation in the emission location (and/or emission direction) in such implementations, there are many ways to efficiently guide or direct the tunable output beam along an intended optical path. For example, such variation may be accommodated for by the collection optics (e.g., numerical aperture of a lens and/or a fiber). Alternatively, as described above, respective optical fibers may be used to guide the pump beam and receive the tunable output beam, and these optical fibers may be accurately aligned to each other (e.g., using v-groove technology, as described above). Thus, instead of translating the semiconductor lasers, they may remain fixed relative to the laboratory frame, and such aligned optical fibers may instead be controllably translated along the semiconductor lasers. In this way, the receiver for the tunable output remains aligned with the pump beam as the pump beam is scanned along the semiconductor laser structures. Semiconductor laser chips 26a1, 26a2, 26a3 . . . 26a(n-1), 26an may be implemented in various ways. For example, the various chips may employ different semiconductor laser device types (e.g., edge-emitting, VCSELs), different active regions (e.g., molar composition, presence or number of quantum wells, etc.), different mirror structure/designs, and/or different material systems (e.g., InP based or GaAs based). As described, in FIG. 2, each of the semiconductor laser chips has a resonance that varies with position in the x-direction. That is, semiconductor laser chip 26a1 has a resonance that varies between wavelengths λ1(1) and λ1(2), semiconductor laser chip 26a2 has a resonance that varies between wavelengths λ2(1) and λ2(2), and generally, semiconductor laser chip 26an has a resonance that varies between wavelengths λn(1) and λn(2). Thus, a widely tunable wavelength range may be provided based on a hybrid approach of using multiple laser chips having different resonance ranges, each laser chip capable of being pumped over its range of resonances by simply varying the location of pump incidence. Although multiple semiconductor laser chips are shown in the embodiment of FIG. 2, in various applications a single semiconductor laser chip may provide a tunable output beam with a sufficient wavelength range.

FIG. 3 illustrates one way in which spatially dependent wavelength variation can be achieved within a semiconductor laser chip, using a tapered, oxidized layer in a VCSEL. More specifically, FIG. 5 depicts a schematic cross-sectional view of a portion of a VCSEL having DBRs 56 and 58 surrounding cavity (e.g., AIGaAs) region 52. The vertical thickness of cavity region 52 is tapered by tapered oxide 54 which may be grown in the cavity (e.g., formed by oxidizing a portion of the AIGaAs cavity material) of a VCSEL structure in accordance with the description in A. Fiore, Y.A. Akulova, J.Ko, E.R. Hegblom, and L.A. Coldren, "Multiple wavelength vertical cavity laser arrays based on postgrowth lateral- vertical oxidation of AIGaAs," Applied Physics Letters, vol.73, no. 3, July 20, 1998, pp. 282-284, which is herein incorporated by reference. It is understood, however, that spatially dependent wavelength variation in VCSELs or other semiconductor lasers may be achieved by numerous other means, such as etching and regrowth, etching, and wafer bonding, etc. For example, in an edge emitting laser, the wavelength variation can be achieved by a variable pitch grating.

In a simple implementation of an embodiment of the present invention using VCSELs as the optically pumped semiconductor laser chips, the VCSEL chips have no refractive index guiding, being completely gain-guided by the pump light. Such an implementation renders the system very easy to assemble, requiring very few critical alignments. In some applications, for example, where extremely high power output is needed and discrete tuning is acceptable, a VCSEL chip may contain an array of index guided lasers along the direction that the resonance varies (i.e, along the x-direction in FIG. 2).

In accordance with an embodiment of the present invention, such arrays of index guided lasers may be designed such that arbitrarily fine channel spacing may still be achieved. For example, FIG. 6 depicts a schematic plan view of the relative location of circular index guides 60a-60h with respect to cavity-oxidation edge 62 for a VCSEL chip. As known by those skilled in the art, the index guiding represented by index guides 60a-60h may be provided in various ways, such as by etching and/or by an oxide layer. More specifically, oxidation for determining the cavity thickness (i.e., normal to the page) of all the index guided lasers occurs from a commonly defined, straight edge, indicated by cavity-oxidation edge 62. Accordingly, since the distance in the cavity-oxidation direction of the index guided laser structures from the straight cavity-oxidation edge 62 varies, each laser will have a different cavity thickness because the oxide thickness varies along this direction. Thus, arbitrarily fine channel spacing may be provided based on the amount of stagger and the oxidation profile.

Another way of obtaining arbitrarily fine spacing with index guiding is to start the cavity oxidation front from a "staircase" edge 66, as shown in FIG. 7. Thus, as in the embodiment of FIG. 6, the laser devices guided by index guides 64a-64e have varying distances from the cavity oxidation edge 66, in the direction of cavity oxidation, and thus have varying cavity thicknesses. Making the "step" of staircase edge 66 small allows the channel spacing to be small. The translation stage would, of course move in the direction of the device stagger.

A compromise between complete index guiding and exclusive gain guiding can be accomplished by index guiding along one dimension. For example, FIG. 8 illustrates a schematic plan view of a portion of a VCSEL in which the cavity is defined by an index guide 70 (e.g., a groove) along one dimension only. Pump spot 72 gain guides along the dimension orthogonal to the index guided direction, the gain guiding being along the direction that the VCSEL is moved relative to the pump beam (e.g., along the x-direction in FIG. 2). Such a design provides some of the efficiency benefits of an index guided device, while also allows for continuous tunability, since there is no interruption of laser gain along the direction in which the cavity resonance varies (i.e., the direction in which VCSEL is moved relative to the pump beam).

It is appreciated that if a semiconductor laser chip includes index guided laser devices, each laser device may itself be made tunable. For example, if each device were a MEMs tunable VCSEL of the type shown in FIG. 1 , then translation may be used to hop among VCSELs, and when a given VCSEL is selected, MEMs tuning may provide a range of tunable outputs. Accordingly, by selectivly pumping different MEMs VCSELs by translating among them, the overall tuning range is represented by the tuning range of one tunable VCSEL multiplied by the number of VCSELs. Translation stage 24 shown in FIG. 2 can be implemented using, for example, using a piezoelectric transducer or a commercially available manual stage. Piezoelectric transducers provide the required range of motion with high tuning speed. Commercially available stages, such as those used with bulk optics, are well suited for lower tuning speed applications. As described, a packaging/substrate/module element 20 may be employed as a common package/substrate/module for mounting semiconductor laser chips 26a1 , 26a2, 26a3 . . . 26a(n-1), 26an, this packaging/substrate/module element 20 being mounted onto translation stage 24b. Such a packaging/substrate/module element 20 may comprise, for example, a ceramic, glass, or semiconductor substrate having a slit or apertures for transmitting, or being optical transparent to, tunable output beam 26b. For various embodiments, packaging/substrate/module element 20 may also include (1) a temperature sensing element (e.g., a thermistor, not shown) sensed by controller 40 via signal line 48, and (ii) possibly also a heating and/or cooling element (e.g., thermoelectric heater/cooler, not shown) controlled by controller 40. As may be appreciated, such a distinct packaging/substrate/module element 20 is optional, as its functionality may be integrally embodied by movable stage 24.

In FIG. 2, controller 40 provides a drive signal to movable translation stage 24 to control its displacement in the x-z plane, thus controlling the wavelength of tunable output beam 26b by controlling the location of the semiconductor laser chips which is impinged upon by pump beam 22b. Controller 40 may also sense the temperature of package/substrate/module 20 via signal line 48 and also possibly control this temperature by providing a temperature control signal via signal line 46 to a heating and/or cooling element (e.g., a thermoelectric heater/cooler) thermally coupled to package/substrate/module 20. If MEMs VCSELs are employed, controller 40 may also provide a signal for controlling the MEMs mirror(s). Controller 40 may be implemented in various ways, including as analog and/or digital circuitry, as a programmed micro-controller, a programmed digital computer (e.g. personal computer or workstation) with data acquisition and control interfaces. It is also noted that implementing controller 40 as a programmed digital computer or microcontroller is well suited for incorporating additional system monitoring and control into an overall, single control system. For example, where such a tunable laser system is implemented in a spectrophotometer, controller 40 may also be operative in executing user interface programs as well as data acquisition and processing operations. In various applications, controller 40 may also embody the control of pump laser 22.

Controller 40 may implement various control algorithms. In a simple implementation of an embodiment according to FIG. 2, controller 40 may simply control the wavelength of tunable output beam 26b in an open loop manner according to a pre-characterized relationship between displacement of translation stage 24 and the wavelength of tunable output beam 26b. Such control is feasible since, for example, a given VCSEL chip should maintain very reproducible wavelength characteristics, provided its temperature is maintained within about a 1 °C window. Even if the temperature of semiconductor laser chips 26a1-26an is not controlled (e.g., by controlling the temperature of package/substrate/module 20), however, since the variation of wavelength with temperature is reproducible and may be pre-characterized, controller 40 may sense the temperature of package/substrate/module 20 via line 48, and open-loop correct for temperature to accurately control the tunable beam wavelength. That is, controller 40 may accurately control the wavelength of tunable output beam 26b without active temperature control by controlling the displacement of translation stage 24 as a function of sensed temperature. For example, if a given wavelength is desired, controller 40 will set a different displacement of translation stage 40 depending on the sensed temperature. Of course, even if controller 40 actively controls the sensed temperature of package/substrate/module 20, it may nevertheless control the displacement of stage 24 according to the sensed temperature to ensure accurate wavelength control. It is also noted that by performing a similar pre-characterization procedure, the control algorithm implemented by controller 40 may also account for variation of the tunable output beam wavelength with pump power, which would vary over temperature if one were attempting to maintain constant power output.

It is further noted that in an alternative embodiment, controller 40 may augment mechanical tuning with thermal tuning for additional wavelength coverage. That is, instead of controlling the temperature of package/substrate/module 20 to be at a given essentially constant value, controller 40 may vary the temperature as needed to reach further desired wavelengths. Additionally, it is noted that in an alternative embodiment, controller 40 may implement closed-loop wavelength control by controlling the displacement of translation stage 24 according to a feedback signal representing the sensed wavelength of tunable output beam 26b. For myriad applications, however, such closed-loop control is not necessary. In accordance with the foregoing description of illustrative embodiments of the present invention, and illustrative variations or modifications thereof, it may be appreciated that the present invention provides many features, advantages and attendant advantages, including the following illustrative features and advantages. For example, although the tuning mechanism is mechanical, the tunable laser does not require any internal moving parts, thus making the device insensitive to mechanical vibration. Also, because an airgap in the optical cavity is not required for tunability, a low-loss and efficient cavity, such as that associated with undoped VCSEL structures, may be employed. Further, the tuning algorithm may be extremely simple, relying only on a one or two dimensional position parameter. In many applications, such a simple tuning algorithm, combined with the lack of moving parts internal to the laser cavity, may make wavelength monitoring unnecessary. An apparent feature illustrated by the foregoing is the wide tunability provided by using multiple semiconductor lasers in the manner described. For example, a single VCSEL chip having a single active region design will typically be limited in tuning range to about 50 nm around 1.55 microns. If semiconductor GaAs/AIGaAs DBR mirrors are used, the limited mirror bandwidth will also limit tuning to about 50 nm. By using multiple VCSEL chips, however, as illustrated in as illustrated in FIG. 2, the overall tuning range can be much greater than the tuning range of any single VSCEL chip. For example, using a 980 nm pump laser, and 10 different VCSEL chips with semiconductor GaAs/AIGaAs DBRs, a 500 nm tuning range (e.g., from 1.1 to 1.6 microns) is practicable with current laser technology. Further developments in blue lasers may make them available as an inexpensive blue pump, thus readily permitting tuning from about 500 nm to 2 microns by using VCSELs with dielectric mirrors (i.e., so they do not absorb the pump light). Such "universal tunable laser" performance can today only be achieved with multi-million dollar free electron lasers, which occupy entire buildings. Of course, if one can generate such wide tuning range, any subsequent optics must be carefully designed in a manner that substantially eliminates chromatic dispersion.

Although the above description provides many specificities, these enabling details should not be construed as limiting the scope of the invention, and it will be readily understood by those persons skilled in the art that the present invention is susceptible to many modifications, adaptations, and equivalent implementations without departing from this scope and without diminishing its attendant advantages. It is therefore intended that the present invention is not limited to the disclosed embodiments but should be defined in accordance with the claims which follow.

Claims

What is claimed is:

1. A tunable laser comprising: a pump laser that emits an optical pump beam of a first wavelength; at least one semiconductor laser chip upon which said optical beam impinges, said at least one semiconductor laser chip being selectively movable relative to said optical pump beam such that said optical pump beam is capable of impinging on different local regions of said at least one semiconductor laser chip, said at least one semiconductor laser chip generating a second optical beam of a second wavelength in response to said optical pump beam, said second wavelength being a non-constant function of the local region upon which said optical beam impinges.

2. The tunable laser according to claim 1 , wherein said at least one semiconductor laser chip is a single semiconductor laser chip having a resonance that varies along at least one spatial dimension of the single semiconductor laser chip.

3. The tunable laser according to claim 1 , wherein said at least one semiconductor laser chip is an array of at least two semiconductor laser chips.

4. The tunable laser according to claim 3, wherein each of the at least two semiconductor laser chips has a resonance that varies along at least one spatial dimension.

5. The tunable laser according to claim 1 , further comprising a movable stage upon which said at least one semiconductor laser chip is mounted.

6. The tunable laser according to claim 5, wherein said movable stage includes a piezoelectric transducer.

7. The tunable laser according to claim 5, wherein said optical pump beam is fixed relative to a laboratory frame of reference, and said movable stage is spatially translated to control said second wavelength according to the local region upon which said optical pump beam impinges.

8. The tunable laser according to claim 7, wherein said movable stage translates in a direction that moves said at least one semiconductor laser chip in a direction having a component that is perpendicular to the direction that said optical pump beam impinges on said at least one semiconductor laser chip.

9. The tunable laser according to claim 1 , wherein said at least one semiconductor laser chip includes a VCSEL chip.

10. The tunable laser according to claim 9, wherein said VCSEL chip has an AIGaAs mirror.

11. The tunable laser according to claim 9, wherein said VCSEL chip has fully oxidized AIGaAs mirrors.

12. The tunable laser according to claim 9, wherein said VCSEL chip has at least one dielectric mirror.

13. The tunable laser according to claim 1 , wherein said pump laser is a semiconductor laser.

14. The tunable laser of claim 13, wherein said first wavelength is in the approximate range of 700 nm to 1100 nm.

15. The tunable laser of claim 14, wherein said second wavelength is in the approximate range of 1100 nm to 1700 nm.

16. The tunable laser of claim 13 , wherein said first wavelength is in the approximate range of 300 nm to 700 nm.

17. The tunable laser of claim 9, wherein the VCSEL chip has a resonance which varies with position on the chip.

18. The tunable laser of claim 17, wherein the resonance variation is provided by a tapered oxide layer.

19. The tunable laser of claim 9, wherein the VCSEL chip has a mode pattern determined substantially by the gain guiding of the pump beam.

20. The tunable laser of claim 9, wherein the VCSEL chip contains one or more index guided device positions.

21. The tunable laser of claim 20, wherein each of the index guided devices is a MEMs tunable VCSEL.

22. The tunable laser of claim 9, wherein which the VCSEL is index-guided in one dimension and gain-guided in another dimension.

23. The tunable laser of claim 1 , further comprising a temperature controller that provides for fine wavelength control of said second optical beam.

24. The tunable laser of claim 23, wherein the translation of said at least one semiconductor laser chip is adjusted to compensate for temperature-induced wavelength changes of said second wavelength.

25. The tunable laser of claim 1, wherein said optical pump beam is delivered to said at least one semiconductor laser chip via a first optical fiber.

26. The tunable laser of claim 25, further comprising a second optical fiber to receive the second optical beam emitted by said at least one semiconductor laser chip.

27. The tunable laser of claim 26, wherein said first and second optical fibers are aligned to each other using a v-groove substrate.

28. The tunable laser of claim 25, wherein said first optical fiber guides said second optical beam emitted by the semiconductor laser chip, said first optical fiber thereby guiding both said pump laser optical beam and said second optical beam.

29. The tunable laser of claim 28, further comprising a dichroic beam splitter that is coupled to said first optical fiber and that separates said first and second wavelengths.

30. The tunable laser according to claim 1 , wherein said at least one semiconductor laser chip includes an edge emitting laser, and said second wavelength is varied according to a variable pitch grating.

31. A wavelength division multiplexed communication link comprising a transmitter and receiver, wherein said transmitter includes the tunable laser according to claim 1.

32. An instrument for optical spectroscopy, comprising an excitation source that includes the tunable laser according to claim 1.

33. A tunable laser, comprising: means for generating a pumping optical beam of a first wavelength; and means for generating a tunable output optical beam having a second wavelength in response to being pumped by said pumping optical beam, said second wavelength depending on the spatial location of the tunable output optical beam means upon which said pumping optical beam impinges.

34. A method for obtaining tunable laser output, comprising: generating a pump optical beam; providing a semiconductor laser chip in the path of said pump optical beam such that said semiconductor laser chip emits a tunable optical beam ; and moving said semiconductor laser chip relative to said pump optical beam such that the wavelength of said tunable optical beam is controlled by the location of said semiconductor laser chip upon which said pump optical beam impinges.

35. The method of claim 34, further comprising the step of controlling the temperature of the semiconductor laser chip for fine wavelength adjustment of the tunable optical beam.

36. The method of claim 34, further comprising the step of controlling the pump power to allow fine wavelength adjustment of the tunable optical beam.

37. The method of claim 34, wherein said moving step includes the step of controlling said location according to a signal representing the temperature of said semiconductor laser chip, thereby controlling said wavelength of said tunable optical beam as a function of temperature.